The invention is based on a hydraulic radial piston engine comprising a lifting ring, in particular a multi-stroke lifting ring (15) which is fixed to the casing, a rotor (18) which is mounted opposite the lifting ring (15) such that it can be rotated about an axis of rotation (19) and has multiple piston seats (35) aligned radially with respect to the axis of rotation (19), multiple pistons (50), of which in each case one is displaceably mounted in a piston seat (35) and which, over at least part of its length, has a guide and sealing cross section that differs from a circular shape and matches a guide and sealing cross section of the piston seats (35), which differs from a circular shape and remains constant as far as the outside of the rotor, and cylindrical rollers (58) which are borne by the pistons (50) in bearing seats (56) and are aligned with their axes of rotation (59) in the direction of the axis of rotation of the rotor (18), via which the pistons (50) can be supported on the lifting ring (15) and which have two end faces (60) that face away from each other and run at right angles to their axis of rotation (59).
Such a hydraulic radial piston engine is disclosed by DE 196 18 793 A1. The rotor is located within the lifting ring and has a large number of piston seats that are aligned radially with respect to its axis of rotation and are open outward toward the lifting ring. In each piston seat there is a piston which bears a cylindrical roller in a bearing seat, said roller resting on a lifting cam of the lifting ring. In the case of use as a motor, a working chamber radially on the inside of the piston is connected to a pressure medium source when a roller is located on an outwardly falling flank of the lifting ring, while the working chamber is relieved of pressure when the roller is located on an inwardly rising flank of the lifting ring. As a result, a torque is produced which leads to a relative rotation between rotor and lifting ring. The magnitude of the maximum torque that can be generated depends on the maximum pressure that can be applied to the pistons and the size of the piston area on which the pressure acts. In order to be able to produce a higher torque with a given overall size of a hydraulic radial piston engine, the intention is therefore to have a large cross section of the pistons. However, a piston can have a large cross section only at a relatively great distance from the axis of rotation of the rotor, since otherwise there would be too little rotor material present between the individual piston seats in order to reliably avoid the rotor tearing. Assuming a constant piston cross section over the entire length of the piston, the guide length for the piston would then be too small.
DE 196 18 793 A1 but, for example, also DE 40 37 455 C1 or EP 0 607 069 B1, also discloses obtaining a large effective piston area and a long guide length in hydraulic radial piston engines by the pistons being formed as stepped pistons and the piston seats accordingly being formed as stepped seats. Each piston seat has a first part seat, which is located entirely radially on the outside in the rotor, and a second part seat which is located closer to the axis of rotation than the first part seat and whose cross section is smaller than the cross section of the first part seat. Each piston has a first piston section, which is guided in a sliding manner in the first part seat, and a second piston section which has a smaller cross section than the first piston section and is guided in a sliding manner in the second piston seat. The clearances located radially further in than the first piston section, namely an annular space which is bounded at right angles to the piston axis by the wall of the first part seat and by the second piston section, and axially by the steps on the piston and in the piston seat, and a completely cylindrical space behind the second piston section are connected fluidically openly to each other, so that the effective pressure area is provided by the large cross section of the first piston section. A piston is guided in the first piston section and at the end of the second piston section, so that the guide length is great. The fluidic connection between the annular space and the space behind the second piston section can be produced by means of bores within the second piston section, by means of a longitudinal groove in the second part seat or else, as shown in EP 0 607 069 B1 and DE 196 18 793 A1, by means of flats on the second piston section, parallel to the piston axis or else located conically with respect thereto.
In the case of the radial piston engine disclosed by DE 196 18 793 A1, the first part seat of a piston seat and the first piston section have a guide and sealing cross section differing from the circular shape. This cross section has two long sides running parallel to the axis of rotation of the roller and two semicircles which connect the long sides to each other. As a result of piston seats and pistons having such an elongate cross section in the direction of the axes of rotation of rotor and rollers, it is possible to obtain a large piston area without enlarging the diameter of the rotor and therefore of the overall radial piston engine. However, an enlargement in the axial direction is necessary if the piston area is to be greater than in the case of a circular piston cross section.
The semicircular sections in the cross section additionally entail the rollers being shorter than the piston seat and the pistons in the direction of their axes of rotation, and it is necessary to secure them using rings revolving around the rotor in their axial position.